Plasma or gaseous discharge display and/or storage apparatus have certain desirable characteristics such as small size, a thin flat display package, relatively low power requirements and inherent memory capability which render them particularly suitable for display apparatus. One example of such known gaseous discharge devices is disclosed in U.S. Pat. No. 3,559,190, "Gaseous Display and Memory Apparatus," patented Jan. 26, 1971 by Donald L. Bitzer et al. Such devices, designated A.C. gas or plasma panels, may include an inner glass layer of physically isolated gas cells or comprise an open panel configuration of electrically insulated but not physically isolated gas cells. In the open panel configuration which represents the preferred embodiment of the instant invention, a pair of glass plates having dielectrically coated conductor arrays formed thereon are sealed with the conductors disposed in substantially orthogonal relationship. Appropriate drive signals are applied to selected groups of conductors, and capacitively coupled to the gas through the dielectric. When these signals exceed the breakdown voltage of the gas, the gas discharges in the selected cell area, and the resulting charge particles, ions and electrons, are attracted to the wall having a potential opposite the polarity of the particle. This resulting wall charge potential opposes the drive signals which produce and maintain the discharge, rapidly extinguishing the discharge and assisting the breakdown in the next sustain signal alternation. Each discharge produces light emission from the selected cell or cells, and by operating at a relatively high frequency in the order of 30-40 kilocycles, a flicker-free display is provided. After initial breakdown, the wall charge condition is maintained in selected cells by application of a lower potential designated the sustain signal which, combined with the wall charge, causes the selected cells to be reignited and extinguished continuously at the applied frequency to maintain a continuous display.
The capacitance of the dielectric layer is determined by the thickness of the layer, the dielectric constant of the material and the geometry of the associated drive conductors. The dielectric material must be an insulator having sufficient dielectric strength to withstand the voltage produced by the wall charge and the externally applied potential. The dielectric should be a relatively good emitter of secondary electrons to assist in maintaining the discharge, be transparent or translucent on the display side to transmit the light generated by the discharge for display purposes, and be susceptible to fabrication without reacting with the conductor metallurgy. Finally, the coefficient of expansion of the dielectric must be compatible with that of the glass plate or substrate on which the dielectric layer is formed.
One material possessing the above characteristics with respect to a soda-lime-silica substrate is lead-borosilicate solder glass, a glass containing in excess of 75 percent lead oxide. In an embodiment constructed in accordance with the teaching of the present invention, a dielectric comprising a layer of lead-borosilicate glass was employed as the insulator. However, degradation or decomposition of the lead oxide at the dielectric surface under the discharge environment produced variations in the electrical characteristics of the gaseous discharge display panel on a cell-by-cell basis. This degradation, resulting primarily from ion bombardment of the dielectric surface, caused the electrical parameters of the individual cells in the gaseous discharge device to vary as a function of the cell history such that over a period of time, the required firing voltage for individual cells fell outside the normal operating range, and the firing voltage varied on a cell-by-cell basis.
In order to avoid degradation of the dielectric surface resulting from ion bombardment in a gaseous discharge device, a refractory high secondary electron emissive material such as magnesium oxide (MgO) is utilized to protect the dielectric surface. The refractory aspect prevents sputtering of the dielectric by ion bombardment, while the high secondary-electron emission aspect permits lower operating voltages. It is known in the art that the breakdown voltage in a gaseous discharge device may be lowered by utilizing a material having a high secondary-electron emission coefficient such as MgO. However, changes in the surface properties, namely the secondary-electron emission coefficient of MgO produced by ion bombardment during the discharge, caused the maximum sustain voltage and the bistable voltage margin of the panel, i.e., the difference between the maximum sustain voltage (V.sub.s max) and minimum sustain voltage (V.sub.s min) required to sustain the lines in the panel, to decrease significantly with panel operating time. During normal panel operation, the maximum and minimum sustain voltages defining the bistable voltage margin of the panel tended to converge over a period of time, effectively reducing the operating margin of the panel below acceptable limits, resulting in reduction of the yield of the panels thus fabricated, thereby significantly raising the panel cost.